This invention relates generally to high performance pulsed laser frequency conversion systems that operate with high power, visible and near-infrared spectral output, narrow spectral linewidths, and freedom to adjust pulse parameters and characteristics.
Fiber lasers have advanced to become economical and efficient high power infrared laser sources. Average optical output powers of tens of kilowatts are currently available in commercial fiber laser systems. FIG. 1 is a schematic of a typical fiber laser system known in the art with a master oscillator fiber amplifier (MOFA) architecture. The master oscillator (also known as the seed laser) emits a low power optical signal that is coupled into the amplifier section through an optical isolator. The optical isolator protects the master oscillator from any light counter-propagating back through the amplifier section. The amplifier section consists of a length of gain fiber that is pumped by one or more pump lasers (typically diode lasers) through a pump coupler. The gain fiber may be multi or single spatial mode, polarization random or maintaining, cladding pumped or core pumped, and may have a variety of dopants (for example Yb, Er, Nd, Pr, etc.) depending on the emission and pumping wavelengths. The pump laser light is absorbed by the dopants in the gain fiber, raising the dopants into an excited state. The emission from the master oscillator is amplified through stimulated emission as it interacts with the excited dopants implanted in the fiber core.
Many variants of the above design are used, including but not limited to multiple gain stages with multiple pumps, the inclusion of various filtering elements, a delivery fiber at the output of the laser, and use of forward and/or backward propagating pumps. Fiber lasers can operate with a wide range of output parameters to satisfy the varying constraints of an application. It is the specifications of the individual fiber amplifier subsystems that determine the output emission. The output emission of a fiber laser can be specified with the average output optical power, peak output optical power, temporal pulse width, center optical wavelength, polarization, spatial mode, and spectral bandwidth. Pumping limitations, gain limitations, optical damage to components, and nonlinear impairments require a unique system design of the elemental blocks of a fiber laser to achieve the desired set of output parameters.
It is desirable to use fiber laser systems to replace current solid state and gas laser systems used in illumination, inspection, and micromachining in the visible and ultraviolet (UV) wavelengths. However, fiber lasers operate in the near infrared (NIR) wavelength region and require nonlinear frequency conversion (FC) to convert the NIR output emission into usable visible and UV wavelengths. Since the efficiency of the nonlinear frequency conversion is typically very low (<1%) at optical powers less than 100 Watts, it is advantageous to use a pulsed laser system with high peak power as the fundamental source. However, many illumination and inspection systems are designed to use continuous wave (CW) laser sources. In order to use a pulsed source in these applications the pulse repetition rate needs to be much higher than the response rate of the system. The term quasi-continuous wave (QCW) is used within this application to describe a pulsed light source of sufficiently high repetition rate so as to appear CW to the application. The repetition rate typically must be greater than 10 MHz to appear QCW in illumination and inspection applications. Thus, in order to gain a significant benefit in peak power, pulse widths much less than 1 ns are required. A common class of lasers that produce a steady stream of sub-nanosecond pulses is mode-locked lasers.
Nonlinear frequency conversion generally requires high peak power, narrow optical bandwidth, linear polarization, and single spatial mode. However, it has not yet been practical to simultaneously satisfy these requirements in a sub-nanosecond, pulsed fiber laser with repetition rate greater than 10 MHz due to nonlinear impairments. In particular Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS), and Self Phase Modulation (SPM) limit the performance of fiber lasers operating in this regime. These nonlinear impairments increase with higher peak intensity in the fiber, with narrower spectral bandwidth, shorter pulse width (especially SPM), and by propagating linear polarized light in a single spatial mode. Examples of mode-locked fiber lasers are known in the art that reduce nonlinear impairments because of the large natural bandwidth of the femtosecond pulses they create, as well as nanosecond pulsed fiber lasers that use a master oscillator with artificially high optical bandwidth to reduce nonlinearities. Similarly, fiber lasers with kilowatt average power are known in the art. These lasers function in continuous wave operation, and do not provide the beam quality, peak power, and polarization properties useful for efficient frequency conversion. Thus, there is a need in the art for improved methods and systems for high performance pulsed laser frequency conversion.